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Mtd 990 Only Runs With Choke Closed - Why?


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#1 SammyRye OFFLINE  

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Posted July 18, 2014 - 06:46 PM

I know this has been asked before, but I couldn't locate the thread, so I thought I'd start a new one.

 

A couple of months ago, I noticed the 990 was runninning with a lot of sputtering.  Last week, I was going out to move some rocks and brush with the rear carry-all, but it would only run with the choke fully closed.  When I would let loose of the choke control, it would start to die.

 

Today I pulled it in the garage (less mosquitos!) and pulled the carb off to check it.  I've checked everything - vent hole in gas cap is open; gas tank is full to the top; checked spark plug gap; replaced fuel filter; made sure everything was in proper adjustment - runs a little better, but it still won't run with the choke open.

 

Anyone else have this happen?  What might be the cause?  And, what is the cure?

 

Thanks for the help - Rich



#2 propane1 ONLINE  

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Posted July 18, 2014 - 07:50 PM

Maybe late timing. Happen to me a few week ago. Make sure your points are set to spec. Just a thought. Now I have never seen a 990 and don't know what engine is in it. I was working on a Kohler engine. Noel
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#3 New.Canadian.DB.Owner OFFLINE  

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Posted July 18, 2014 - 07:50 PM

Closing the choke increases the vacuum inside the carb, and thus draws more gas from the carb.  You likely have a blocked venturi or a leaking carb-to-block gasket. 


Edited by New.Canadian.DB.Owner, July 18, 2014 - 07:51 PM.

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#4 bread320i OFFLINE  

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Posted July 18, 2014 - 08:11 PM

Your engine is starving for fuel. Check you fuel sediment bowl, and fuel filter. If flow is fine clean carburetor.

A quick and easy test. Spray gas through a Windex bottle into carburetor. If your engine runs fine you have a fuel issue. No change you have an ignition issue.


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#5 SammyRye OFFLINE  

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Posted July 18, 2014 - 09:32 PM

Thanks for the replies, guys!

 

Should have mentioned this, but the moter is a B&S 16 hp single cylinder.

 

Cleaned the carb today - there wasn't anything in it - clean as could be; Checked the nozzle, float valve and float - all looked like they were just put in;

Put a new fuel filter in-line; No sediment bowl;

Made sure the gas was full;

Checked the gas cap vent hole - no blockage;

Checked the gas line flow - seems good;

No condensor or points - converted to electronic ignition;

Timing?  Only if the key sheared, and I checked that;

 

Not sure what else to check?

 

Thanks, Rich

 


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#6 EricFromPa ONLINE  

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Posted July 19, 2014 - 12:46 AM

No condensor or points - converted to electronic ignition

 

That will be your problem.If your flywheel has more than 1 Magnet.

 

It is a 3 magnet flywheel?

You can't use an electronic ignition coil on a flywheel with multiple magnets.

 

If it is a single magnet flywheel and it ran after you switched it over to solid state ignition I would either suspect Blown head gasket or sticking exhaust valve.


Edited by EricFromPa, July 19, 2014 - 12:50 AM.

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#7 JDBrian OFFLINE  

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Posted July 19, 2014 - 05:09 AM

Have a look at the spark you are getting and if it looks nice and snappy then check the timing. A poor or badly timed spark can cause general sick engine syndrome that can look like a fuel problem. Since you have checked the fuel system it's time to look elsewhere. I've seen this happen with points before. 


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#8 EricFromPa ONLINE  

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Posted July 19, 2014 - 05:38 AM

Spark will be strong with a 3 magnet flywheel but it will fire Way to advanced for it to run properly.

 

I tried to use an electronic ignition coil on my FourteenHundred and it would not run with the choke off and it was popping through the carb quite a bit.I tried a Nova II ignition module with the original coil and it did the same exact thing.

 

I ended up putting the points ignition back in and bought a points ignition coil.My coil had been chewed up by critters under the flywheel.


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#9 SammyRye OFFLINE  

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Posted July 19, 2014 - 10:18 AM

Thanks much!  Didn't know you couldn't use the electronic ignition only on a 1 magnet flywheel.

 

Okay - back to teraing the front of this thing off again - UGH!  Good thing I have new points and condensor handy.  I'll check the timing to make sure that's okay, and change back to the breaker and condensor and see what happens.

 

I'll post results on here later.

 

Thanks again - Rich



#10 VTXrider ONLINE  

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Posted July 19, 2014 - 02:38 PM

Hi Sammy. I hate taking the front end off.

 

The flywheel on the 16 is a single magnet as far as I know.

 

Did your machine run ok after the ignition mod? If so, maybe it could still be the carb. I have had the same problems with 3 different 990s and most of the time it is the points needing adjustment or another condenser. Other times it is the float sticking. 

 

When the ignition is working correctly, I have been able to keep the engine running with a tablespoon of gas pouring into the carb. Also with a spray bottle squirting onto the carb. Not very safe to do so wear glasses at least. Sometimes the carb comes loose and leaks air. It will still run but needs choke. Check the 2 gaskets on the manifold elbow. 

 

Bread320i suggested tests should narrow it down.


Edited by VTXrider, July 19, 2014 - 02:45 PM.

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#11 SammyRye OFFLINE  

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Posted July 19, 2014 - 05:04 PM

VTXrider - Yes, I hate taking the front end off the 990's as well - very time consuming!

 

Anyway, what I had put on there was the Stens Mega Fire II Ignition Module.  When I first put it on, the tractor ran fine.  It's been slowly getting worse.  Ran it all winter with the snow thrower on, but it was running poorer towards the end.  Is it a carb problem? Don't know that it isn't for sure.  I had the carb totally apart yesterday and checked everything, but it's kinda hard to check each little orifice.  There was no crud build up anywhere, so I discounted the fuel problem.

 

You mentioned the two gaskets on the maniford elbow.  There was one on the carb, but none on the elbow to engine.  There is a metal plate that goes in between the manifold and the block, and I didn't know if putting a gasket in between the elbow and that plate would make much difference.  I do have a spare gasket that I can add there.

 

EricFromPA mentioned not being able to use electronic ignition with a multiple magnet flywheel - mine only has one magnet, so I should be able to use the mega fire rather than the condenser and points.

 

With an electronic ignition, how do I check the timing?  Looking at the front of the flywheel, there is an arrow on the right side of the magnet - what should that line up to for proper timing?

 

These old MTD's / Briggs are frustrating, but I love this big ole tractor - gotta get her running again!

 

Thanks for all the help - Rich


Edited by SammyRye, July 19, 2014 - 05:05 PM.

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#12 propane1 ONLINE  

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Posted July 19, 2014 - 06:19 PM

I was going to mention to check the spark plug but you say it was running fine for a while.  Bad engine to body ground? Bad grounds can do funny things.  Noel


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#13 EricFromPa ONLINE  

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Posted July 19, 2014 - 09:50 PM

Oh ok Sorry I didn't know that it was a single magnet.My FourteenHundred is a bit older and the big Gilson 16 Hydro is a year or 2 newer and has factory electronic ignition.

 

Those old carbs are really good once you get them dialed in.I like them alot more than those darn Walbro carbs.

 

 

I did have 1 big briggs that the Coil Bracket came loose on and was loosing ground but that was easy to spot.It was sparking under the flywheel.


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#14 MH81 ONLINE  

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Posted July 20, 2014 - 07:08 AM

I think there should be a gasket at every metal to metal surface on the intake.
Like above, check for good spark, but if it ran ok all winter and then started to mess up slowly, it sounds like gas or condenser... And you don't have a condenser.
Also, the carbs can be a PITA and each one of those holes do something, likely a glob of new gas mung has found a new home in a passage. I will get the engine up to operating speed and then slap my palm on the intake. Repeat several times. Let the vacuum of the engine attempt to dislodge it.
Works often enough to give it a whirl, if not you will have to do a more thorough cleaning.
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#15 VTXrider ONLINE  

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Posted July 20, 2014 - 07:51 AM

There should be thin gaskets on each side of that plate. At least mine has them. A small air leak on these old carbs can usually be adjusted to compensate. Like for the throttle butterfly shaft wear. The newest carb I have, has sharp reaction to adjusting the high and low speed screws, the older carbs not so sharp. There is an interaction between the two adjustments and finding the right combination takes some fiddling. 

 

On page 22 of the B&S service manual shows the alignment procedure for the flywheel and magneto. You can DL the manual from here if you do not have it. It is a large PDF file but well worth having.

http://1drv.ms/1nWTc39 

 

Anyone is welcome to DL it.


Edited by VTXrider, July 20, 2014 - 07:58 AM.

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